HYDROCARBON MIGRATION AND THERMAL DEGRADATION OF MONTEREY FORMATION OIL IN SANTA MARIA BASIN, OFFSHORE CALIFORNIA

High heat flow in the offshore Santa Maria Basin released hydrocarbon gas from Monterey Formation source rocks. Temperature changes induced by geothermal heating during burial enabled thermal cracking of the oil to lighter-end hydrocarbon gases (C1-C4) that migrated as gas plumes into shallow burial depths. Drill stem test records using American Petroleum Institute (API) oil gravity designation indicate that Monterey Formation oil gravities range from 3° to 35°. Oil gravities are related to zones of shallow gas-charged sediments (0-3000 ft subsea), variable geothermal gradients ranging 1.7°F/100 ft to 3°F/100 ft, and temperatures up to 250°F in the Monterey Formation.

API gravity of Monterey Formation sourced oil ranges from 5° to 10° off Point Conception and south of the Amberjack high. This correlates with shallow gas-charged sediments, lower concentrations of C1-C4, and with seafloor expressions interpreted as 350+ tar mounds and 150+ gas-vent craters. Variability of the API oil gravity off Point Arguello and Point Pedernales ranges 15° to 35°, and ranges from 3° to 15° between Purisima and Point Sal. Both areas are associated with near-surface to shallow gas-charged sediments with elevated concentrations of C1-C4, and 1300+ gas-vent craters. Gas plumes migrate upward along faults, anticlinal folds, and steeply dipping beds into shallow sediment from depth. These are linked to high geothermal gradients and controlled by active tectonics. Gas deeply sourced in the Monterey Formation migrates vertically into near-surface sediment, and escapes through the seafloor, forming gas-vent craters. Geochemical trends, piston core samples, SNIFFER and high-resolution geophysical surveys provide support of migrating and seeping gas as thermogenic in origin.